1) You have to obtain the .js file (aka, javascript file) (usually by clicking a download button on a webpage)
2) Place the .js file in your game folder. (the folder containing the files for the project you want the plugin to be used with, usually found initially in thisPC/documents/games, I moved mine to my desktop for easy access)
3) open your game folder, open the folder js, open the folder plugins, add your .js file. (your plugin)
3) open the RPG Maker editor software and look for the little puzzle piece on the top right of the window. Click it.
4) double click an empty space in the resulting window. Click the downward arrow on the dropbox in the widow that appears and double click the plugin you put in your plugin folder. Select turn on. Set the parameters if needed.
These are the basic first steps for a standalone windows version of RPG Maker MV. Anything further would require that you state the specific plugin(s) you're trying to use.